Dr Rowan Williams' interview with the BBC's World at One, in which he called for greater public recognition of some aspects of sharia law, is entirely characteristic. It is the product of deep thought; reasonable, thought-provoking, and in parts quite astonishingly silly.

Let's start with the good bits, which will almost certainly be ignored in a general howl of execration. He wants no part of the crimes and follies which represent sharia law in the public imagination: no stonings, no amputations; in his own words: "Nobody in their right mind I think would want to see in this country the kind of inhumanity [nor] the extreme punishments, the attitudes to women ... that sometimes appears to be associated with the practice of the law."

What he means by sharia is something altogether more benign, and closer to the kind of arrangements already made for talmudic law in Britain. He wants disputes between believers to be regulated, where both agree, by religious laws. It is hard to construct a principled reason why this privilege should be denied to Muslims when it is extended to Orthodox Jews, whose networks of religious courts are perfectly compatible with English law and generally recognised.

His vision of the accommodation of religious law within a secular framework can sound like something no more threatening than Anglican primary schools: a sort of game that the players are free at any time to leave: "I think it would be quite wrong to say that we could ever license, so to speak, a system of law for some community which gave people no right of appeal, no way of exercising the rights that are guaranteed to them as citizens in general," he said on the World at One.

And there is a common-sense argument which says that in matters like marriage and divorce, which can't simply be seen as private arrangements involving only the adults concerned, then it is important that the communities involved should incorporate their own rituals. After all, the modern British wedding, even when it is blessed by the church, incorporates a lot of secular elements. Why shouldn't the secular law incorporate gracefully religious sensibilities?

In the archbishop's vision of a kinder, more feminist, sharia - a vision shared by some reforming Muslims - Islamic law would become an instrument of the liberation of women from their cultural shackles, and the British state would ease the process along. Feminism would become some kind of Islamo-British hybrid, with roots in both systems. Nor is he alone in hoping for this outcome. Something similar lies behind some of the government's recent encouragement of Sufism as a counterbalance to more violent and politicised forms of Islam. If it is true, as the archbishop and his allies argue, that many of the most obnoxious practices in modern Islamic societies are much more cultural than essential to Islam, then sharia courts might just become powerful allies in the struggle against forced marriages, female genital mutilation, and other evils.

None the less, the recognition of sharia law isn't just the same as extending the privileges already extended to the Church of England, and it isn't in a way which makes his colleague, the Bishop of Rochester, look more sensible than he is. That, perhaps, is something that only a very clever man could do. The law of the Church of England, because it is an established church, is enforceable, ultimately, by the state. I once very nearly had a photographer friend sent to prison because I encouraged him to take a picture of some Anglican dignitaries in a church who were considering whether the hideous murals around its interior, which had been painted by a previous vicar's boyfriend, could properly be removed. It turned out that to photograph these deliberations was to commit contempt of court, just as much as if my friend had whipped out his Leica at the Old Bailey and he had to make a very grovelling apology to stay on the streets (or, as he drove, the pavements).

No one would be sent to jail for photographing a sharia court, or even the Orthodox Jewish Beth Din. But in this apparently trivial difference lies the really important point about the relationship between faith and the state. A country can have at most one established religion, whose judgments the state will enforce. Dr Williams, characteristically, is interested in the arguments over what sharia law actually says. The rest of the country is more interested in whether and how it might be enforced. Only if Islamic law can be reduced to a game played between consenting adults can it be acceptably enforced in this country; and that's not, I think, how it is understood by its practitioners. Let's hope I'm wrong.